Flexible unbonded pipes of the present type are for example described in the standard “Recommended Practice for Flexible Pipe”, ANSI/API 17 B, fourth Edition, July 2008, and the standard “Specification for Unbonded Flexible Pipe”, ANSI/API 17J, Third edition, July 2008. Such pipes usually comprise an inner liner also often called an inner sealing sheath or an inner sheath, which forms a barrier against the outflow of the fluid which is conveyed in the bore of the pipe, and one or more armoring layers. In general flexible pipes are expected to have a lifetime of 20 years in operation. Examples of unbonded flexible pipes are e.g. disclosed in U.S. Pat. No. 6,454,887, U.S. Pat. No. 6,415,825, U.S. Pat. No. 6,123,114 and U.S. Pat. No. 5,601,893.
The term “unbonded” means in this text that at least two of the layers including the armoring layers and polymer layers are not bonded to each other. In practice the known pipe normally comprises at least two armoring layers which are not bonded to each other, neither directly nor indirectly via another layer.
The inner sealing sheath is the innermost sealing sheath of the pipe.
For many applications a pipe of the above type will need to fulfill a number of requirements. First of all the pipe should have a high mechanical strength to withstand the enormous forces it will be subjected to during transportation, deployment and operation. The internal pressure (from inside of the pipe and outwards) and the external pressure (from outside of the pipe) are usually very high and may vary considerably along the length of the pipeline, particularly when applied at varying water depths. For many applications, and in particular for use at water depths where it will be subjected to very high external pressure it is required that the unbonded flexible pipe comprises an internal armor layer arranged inside the inner sealing sheath to prevent collapse of the pipe. Such internal armor layer is usually referred to as a carcass and is usually in the form of helically wound and interlocked wires.
However, due to the heat of the fluid often transported in such pipes there have been problems since the influence of the carcass to the inner sealing sheath when subjected to increased pressure and heat has shown to be very damaging. When a notch was formed in the inner sealing sheath, crack propagation from the notch resulted in weakening of the inner sealing sheath and ultimately the inner sealing sheath would leak.
An attempt to solve this problem has been provided in U.S. Pat. No. 4,706,713, by winding a protective sheath around the carcass prior to extrusion of the inner sealing sheath thereon.
In U.S. Pat. No. 6,843,279 it is suggested to wind a tape with a winding angle of less than 35 degrees between the carcass and the inner sealing sheath.
The application of a protective sheath between the carcass and the inner sealing sheath as described in the above prior art document has shown to alleviate risk of damaging the inner sealing sheath due to notch sensitivity for some applications. However, for applications where the unbonded flexible pipe will be subjected to very high external pressure and/or where the fluid transported is very hot, the problems of inner sealing sheath damage still need to be solved.